1. Field of the Invention
The invention relates generally to the sulfonation of fatty acid esters and derivatives thereof and, more particularly, the invention relates to methods of sulfonation of fatty acid esters and of bleaching and neutralizing the resulting sulfonated product.
2. Description of Related Technology
The sulfonates of fatty acids and fatty acid derivatives are commercially valuable as wetting agents and detergent components due to their excellent surface-active properties and resistance to hard water. A sulfonation process which includes reacting excess gaseous sulfur trioxide with fatty acids or fatty acid derivatives to produce a sulfonic acid is well known in the art. Because the sulfonic acid containing product resulting from such a process is typically dark in color and therefore undesirable for commercial use, it is also known to bleach the product, for example, by treatment with hydrogen peroxide. The bleached product is then typically neutralized with sodium hydroxide.
The known bleaching processes are limited in several ways. For example, prior experiments teach that the bleaching process should be performed at a temperature of 90.degree. C. or less. Typically, a bleaching temperature between about 70.degree. C. and about 80.degree. C. has been utilized. For example, in an experiment where methylester sulfonic acid was reacted with hydrogen peroxide in stainless steel equipment, heating the bleaching system above 75.degree. C. resulted in product having a darker color than product bleached at lower temperatures. Operation at higher temperatures resulted in a rapid exotherm and the total consumption of hydrogen peroxide. Once all the hydrogen peroxide was consumed, the color of the acid increased with time.
The prior art teaches that the sulfonation and bleaching processes may be improved by adding an aliphatic alcohol to the reaction mixture. For example, Ogoshi, et al., U.S. Pat. No. 3,997,575 (Dec. 14, 1976) discloses utilizing up to about twenty weight percent alcohol in a sulfonic acid bleaching process. Although the use of a higher percentage of alcohol would seem to be desirable because the alcohol decreases the viscosity of the reaction mixture, thereby guarding against undesirable localized temperature increases which occur in a more viscous mixture, it has generally been taught that a higher amount of alcohol is undesirable for several reasons, including difficulties in removing free alcohol from the neutralized product and the unpleasant odor of the alcohol which necessitates deodorization of the product.
The known bleaching processes (and corresponding neutralization processes) often produce undesirable by-products in addition to the desirable lighter-colored reaction product. For example, the neutralized product may include unreacted hydrogen peroxide, oxygen from the decomposition of hydrogen peroxide, dimethyl ether or other ethers and possibly organic peroxides. These materials can present hazards in downstream storage and processing.
Another challenge faced in the preparation of sulfonates of fatty acids and fatty acid derivatives is to produce a highly active sulfonate paste (i.e. having greater than 55 wt. % of the surface-active neutralized sulfonate) and a low yield of undesirable by-product di-salt (less than 6 wt. %). In the past, neutralization with metal hydroxides and water were easily performed at active concentrations of up to about 35 wt. %, but attempts to produce higher active concentrations also produced a very high di-salt yield. Also, previously, the sensitivity of the continuous neutralization process to factors such as alcohol concentration, neutralization temperature and neutralization pH was not known.